Maya HumanIK to Blender
As a general rule, rigs do not import from one application to another. You may be able to transfer bones or keyframes in some cases using formats like FBX, but nothing more than that. Things like IK, FK/IK switches, constraints, links to shapekey(blendshape) controls and so on are application specific. The only way to transfer them is to have a purpose-built script on the import side that recreates all of these in the new app. One does not exist for HumanIK in Blender that I am aware of.

If you have a Maya character that you want to add to a Blender scene, you are much better off either posing/animating it in Maya and sending over an Alembic cache, or re-rigging the character from scratch in Blender. While an FBX file may be able to transfer the bone structure, tearing it down to something you could make a Blender rig out of would likely be more work than just making a new Blender rig, or using an existing Blender rig or tool (such as Rigify).